ArF laser with low pulse energy and high rep rate

ABSTRACT

A reliable modular production quality ArF excimer laser capable of producing laser pulses at repetition rates in the range of 3,000 to 4,000 Hz or greater with pulse energies in the range of about 2 mJ to 5 mJ or greater with a full width half, maximum bandwidth of about 0.4 pm or less and dose stability of less than 0.4 percent. Using this laser as an illumination source, stepper or scanner equipment can produce integrated circuit resolution of 0.10 μm (100 nm) or less. Replaceable modules include a laser chamber; a modular pulse power system; and a line narrowing module. For a given laser power output, the higher repetition rate provides two important advantages. The lower per pulse energy means less optical damage and the larger number of pulses for a specified illumination dose means better dose stability.

This Application is a Continuation-In-Part of Ser. No. 09/041,474,Reliable, Modular, Production Quality Narrow Band KrF Excimer Laser,filed Mar. 11, 1998, now U.S. Pat. No. 5,991,324, all of which areincorporated herein by reference.

This invention relates to lasers and in particular to narrow-band ArFexcimer lasers.

BACKGROUND OF THE INVENTION

Krypton-Fluoride (KrF) excimer lasers are currently becoming theworkhorse light source for the integrated circuit lithography industry.The KrF laser produces a laser beam having a narrow-band wavelength ofabout 248 nm and can be used to produce integrated circuits withdimensions as small as about 180 nm. The Argon Fluoride (ArF) excimerlaser is very similar to the KrF laser. The primary difference is thelaser gas mixture and a shorter wavelength of the output beam.Basically, Argon replaces Krypton and the resulting wavelength of theoutput beam is 193 nm. This permits the integrated circuit dimensions tobe further reduced to about 120 nm. A typical prior-art KrF excimerlaser used in the production of integrated circuits is depicted in FIG.1 and FIG. 2. A cross section of the laser chamber of this prior artlaser is shown in FIG. 3. A pulse power system 2 powered by high voltagepower supply 3 provides electrical pulses to electrodes 6 located in adischarge chamber 8. Typical state-of-the art lithography lasers areoperated at a pulse rate of about 1000 to 2000 Hz with pulse energies ofabout 10 mJ per pulse. The laser gas (for a KrF laser, about 0.1%fluorine, 1.3% krypton and the rest neon which functions as a buffergas) at about 3 atmospheres is circulated through the space between theelectrodes at velocities of about 1,000 inches per second or greater.This is done with tangential blower 10 located in the laser dischargechamber. The laser gases are cooled with a heat exchanger 11 alsolocated in the chamber and a cold plate (not shown) mounted on theoutside of the chamber. The natural bandwidth of the excimer lasers isnarrowed by line narrowing module 18. Commercial excimer laser systemsare typically comprised of several modules that may be replaced quicklywithout disturbing the rest of the system. Principal modules include:

Laser Chamber Module,

Pulse Power System with: high voltage power supply module, commutatormodule and high voltage compression head module,

Output Coupler Module,

Line Narrowing Module,

Wavemeter Module,

Computer Control Module,

Gas Control Module,

Cooling Water Module

Electrodes 6 consist of cathode 6A and anode 6B. Anode 6B is supportedin this prior art embodiment by anode support bar 44 which is shown incross section in FIG. 3. Flow is clockwise in this view. One corner andone edge of anode support bar 44 serves as a guide vane to force airfrom blower 10 to flow between electrodes 6A and 6B. Other guide vanesin this prior art laser are shown at 46, 48 and 50. Perforated currentreturn plate 52 helps ground anode 6B to the metal structure of chamber8. The plate is perforated with large holes (not shown in FIG. 3)located in the laser gas flow path so that the current return plate doesnot substantially affect the gas flow.

A peaking capacitor comprised of an array of individual capacitors 19 ischarged prior to each pulse by pulse power system 2. During the voltagebuildup on the peaking capacitor, two preionizers 56 weakly ionize thelasing gas between electrodes 6A and 6B and as the charge on capacitorsreach about 16,000 volts, a discharge across the electrode is generatedproducing the excimer laser pulse. Following each pulse, the gas flowbetween the electrodes of about 1 inch per millisecond, created byblower 10, is sufficient to provide fresh laser gas between theelectrodes in time for the next pulse occurring one millisecond later.

In a typical lithography excimer laser, a feedback control systemmeasures the output laser energy of each pulse, determines the degree ofdeviation from a desired pulse energy, and then sends a signal to acontroller to adjust the power supply voltage so that energy ofsubsequent pulses are close to the desired energy. In prior art systems,this feedback signal is an analog signal and it is subject to noiseproduced by the laser environment. This noise can result in erroneouspower supply voltages being provided and can in turn result in increasedvariation in the output laser pulse energy.

These excimer lasers are typically required to operate continuously 24hours per day, 7 days per week for several months, with only shortoutages for scheduled maintenance. One problem experienced with theseprior-art lasers has been excessive wear and occasional failure ofblower bearings.

A prior art wavemeter utilizes a grating for coarse measurement ofwavelength and an etalon for fine wavelength measurement and contains aniron vapor absorption cell to provide an absolute calibration for thewavemeter. This prior art device focuses the coarse signal from thegrating onto a linear photo diode array in the center of a set offringes produced by the etalon. The center fringes produced by theetalon are blocked to permit the photo diode array to detect the coarsegrating signal. The prior-art wavemeter cannot meet desired speed andaccuracy requirements for wavelength measurements.

A need exists in the integrated circuit industry for a modular,reliable, production line quality ArF excimer laser in order to permitintegrated circuit resolution not available with KrF lasers.

SUMMARY OF THE INVENTION

The present invention provides a reliable modular production quality ArFexcimer laser capable of producing laser pulses at repetition rates inthe range of 3,000 to 4,000 Hz or greater with pulse energies in therange of about 2 mJ to 5 mJ or greater with a full width half, maximumbandwidth of about 0.4 pm or less and dose stability of less than 0.4percent. Using this laser as an illumination source, stepper or scannerequipment can produce integrated circuit resolution of 0.10 μm (100 nm)or less. Replaceable modules include a laser chamber; a modular pulsepower system; and a line narrowing module. For a given laser poweroutput, the higher repetition rate provides two important advantages.The lower per pulse energy means less optical damage and the largernumber of pulses for a specified illumination dose means better dosestability.

Important improvements have been provided in the pulse power unit toproduce faster charging. These improvements include an increasedcapacity high voltage power supply, an improved commutation module thatgenerates a high voltage pulse from the capacitors charged by the highvoltage power supply and amplifies the pulse voltage 23 times with avery fast voltage transformer having a secondary winding consisting of asingle four-segment stainless steel rod. A novel design for thecompression head saturable inductor (referred to herein as a “pots andpans” design) greatly reduces the quantity of transformer oil requiredand virtually eliminates the possibility of oil leakage which in thepast has posed a hazard.

Improvements in the laser chamber permitting the higher pulse rates andimproved bandwidth performance include the use of a single preionizertube.

Improvements in the resonance cavity of preferred embodiments of thepresent invention include a line narrowing module with CaF prism beamexpanders and a grating specially coated for UV damage resistance.Preferred embodiments comprised output couplers having substantiallyincreased reflectivity over prior art designs.

A newly designed wavemeter including a computer processor programmedwith an algorithm for controlling wavelength measurements and computingwavelengths at rates sufficient for feedback control of the wavelengthof the output laser beam at rates of 3,000 Hz or faster. In a preferredembodiment a vapor cell with platinum vapor providing a referenceabsorption line for system calibration.

Other embodiments of the present invention include ceramic bearings.Optionally magnetic bearings may be utilized. Reaction forces on thebearings may be reduced by providing an aerodynamic contour on the anodesupport bar. Other improvements include use of acoustic baffles forlaser chambers producing disruptive acoustic shock waves.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing of a prior art commercial excimer lithography laser.

FIG. 2 is a block diagram showing some of the principal elements of aprior art commercial excimer lasers used for integrated circuitlithography.

FIG. 3 is a drawing of the laser chamber of the FIG. 2 laser.

FIG. 4 is a drawings of a preferred embodiment of the present invention.

FIG. 5 is a drawing showing a blower drive unit including magneticbearings.

FIGS. 6 and 6A are cross section drawings of laser chambers of preferredembodiments of the present invention.

FIG. 7 is a drawing showing features of a preferred preionizer tube.

FIG. 8A is a block diagram of a pulse power system of the preferredembodiment of the present invention.

FIG. 8B is a simplified circuit diagram of the above preferredembodiment.

FIG. 8C is a combination block diagram, circuit diagram of a highvoltage power supply which is part of the above preferred embodiment.

FIG. 8D is a prospective assembly drawing of a pulse transformer used inthe above preferred embodiment.

FIG. 8E is a drawing of a primary winding of a pulse transformer used inthe above preferred embodiment.

FIGS. 8F1, 8F2 and 8F3 are time line charts showing pulse compressionusing the above preferred embodiment operating at 1000 Hz.

FIGS. 8G1 and 8G2 are drawing showing two views of a saturable inductor.

FIGS. 8H1 and 8H2 shows the mounting of a compression head in apreferred embodiment.

FIG. 8I shows a preferred modification to the FIG. 8B configuration.

FIG. 8I(1) shows an advantage of the FIG. 8I circuit over the FIG. 8Bcircuit.

FIGS. 9A and 9B are drawings describing a preferred heat exchangerdesign.

FIG. 10 is a drawing showing features of an improved wavemeter.

FIG. 10A is a graph of an ArF excimer laser broadband spectrum.

FIG. 10B is an ArF narrowband spectrum.

FIG. 10C shows the relationship between tuning mirror position andoutput wavelength for an ArF excimer laser.

FIG. 10D is a block diagram showing the principal elements forcontrolling wavelength of an excimer laser.

FIG. 10E depicts a 1024-pixel photo diode array.

FIG. 10F describes the light patterns on the FIG. 5 photo diode arrayused for making coarse and fine wavelength measurements.

FIGS. 10G1 and 10G2 are views of an etalon.

FIG. 10H is a drawing of an etalon assembly.

FIG. 10J is a graph showing a calibration procedure.

FIG. 10K is a drawing of a vapor cell.

FIGS. 11A and 11B are graphs describing the functioning of the FIG. 10wavemeter.

FIGS. 12A through 12E show various anode support bar designs.

FIG. 13 describes a preferred enclosure cooling system.

FIGS. 14A, 14B and 14C show preferred blower blade structure designs.

FIGS. 15A and 15B show grating cross section.

FIGS. 16A and 16B SHOW ALTERNATIVE POWER SUPPLY CIRCUITS.

FIGS. 17A, 17B, 17C, 18 and 19 show test data using a preferredembodiment of the present invention.

FIG. 20 shows a high voltage feedthrough with a single-piece insulator.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS First Preferred Embodiment

A preferred embodiment of the present invention can be described byreference to the drawings.

Modular Laser Design

A front view of a preferred embodiment of the present invention is shownin FIG. 4 respectively. This drawing emphasizes the modular nature oftheir particular invention which allows very quick replacement ofmodules for repair, replacement and maintenance. The principal featuresof this embodiment are listed below corresponding to the referencenumbers shown on FIG. 4.

201 Laser enclosure

202 Gas module

203 Cooling water supply module

204 AC/DC distribution module

205 Control module

206 Line narrowing module

207 Compression head

208 High voltage pulse power supply module

209 Commutator module for pulse power supply

210 Metal fluoride trap

211 Laser chamber

213 Wavemeter module

214 Automatic shutter

216 Output coupler

217 Blower motor

218 Metal fluoride trap power supply

219 Status lamp

220 24 volt power supply

221 Chamber window

222 Gas control flexible connection

224 Vent box

Higher Repetition Rate Lower Pulse Energy

As compared to prior art ArF lasers, a first preferred embodiment of thepresent invention a substantial reduction in the per pulse energy andcompensater with a correspondingly higher pulse repetition rate. Forexample, to produce an output of 10 watts at a repetition rate of 1000Hz each pulse would have energy of 10 mJ. To produce 10 watts at 2000 Hzthe pulse energy would be 5 mJ. In this preferred embodiment the perpulse energy is reduced to about 3.3 mJ and the repetition rate isincreased to 3000 Hz. The present invention also is a substantialreduction in the laser beam bandwidth.

Important improvements (as compared to prior art modular laser system)have been made to permit reliable, consistant laser operation at thesegreatly increased repetition rates while controlling laser parameters toextremely tight specification. These improvements are discussed below.

Preferred Embodiment

A preferred embodiment of the present invention is an improved versionof the laser described in FIGS. 1, 2 and 3. This preferred embodimentincludes the following improvements:

1) A single tube larger preionizer replaces the prior-art combination ofa two-tube preionizer to provide improved efficiency, betterpreionization and improved laser gas flow between the electrodes;

2) A one-piece silicon free machined fan blade;

3) Acoustic baffles are provided to minimize adverse effects of acousticshock waves resulting from the electric discharges;

4) The solid-state pulse power system has been modified to producefaster rise time, providing more consistent pulses, and improved laserefficiency at higher voltages;

5) More precise control of the charging voltage of the pulse powersystem;

6) The reflectivity of the output coupler has been increased to about30%, substantially decreasing the bandwidth of the output pulse;

7) Fused silica prisms have been replaced with CaF prisms to providemuch better thermal stability;

8) MgF₂ coated grating;

9) An improved wavemeter providing much more precise measurement ofnominal wavelength and bandwidth is provided;

10) Improved wavemeter by using an elastomer free etalon design and anew diffuser; and

11) A computer controller programmed with a new algorithm providing amuch improved control of pulse energy and burst energy.

Chamber Improvements Single Preionizer Tube

As shown in FIG. 6, a single larger preionizer tube 56A has replaced thetwo-preionizer tubes 56 shown in FIG. 3. The single tube preionizer isfabricated in accordance with the description in U.S. Pat. No.5,719,896, issued Feb. 17, 1998, which is incorporated herein byreference. Applicants have discovered that one preionizer tube is notonly sufficient, but very surprisingly provides improved performanceover the two-preionizer design. In this embodiment the preionizer islocated upstream of the electrodes. Applicants do not fully understandthe reason for the improved performance. Applicants have determined thatthe one tube preionizer improves in the pulse-to-pulse stability byproviding improved spacial stability of the discharge.

Referring now to FIG. 7, this preionizer utilizes an integrated tubedesign, having bushing element 180 with anti-tracking grooves 170incorporated therein as an integral component of the tube. The diameterof the rod portion 145 and the OD of the bushing portion 180 of thepreionizer is ½ inch. The inside conductor rod 146 has a diameter of{fraction (7/37)} inch and the connecting wire extending through thebushing section to make a ground connection is about {fraction (1/16)}inch diameter. Prior preionizer tube designs utilized a two-diameterdesign, with the rod portion at about ¼ inch diameter and the bushingsat about 1 inch diameter. This necessitated, for manufacturing purposes,a bonding process to join the bushing component with the tube component.The constant diameter, thicker tube design is contrary to conventionaldesign rules, which would predict a reduction in ionization due to lowercapacitances. In most designs, the tube thickness is dependent upon thedielectric strength of the material selected. Those skilled in the artwill recognize that the prior art conventional preionizer tube designtechnique is to select a material with the highest dielectric strengthand determining a wall thickness to match this capacity. For example, asapphire material is known to have a dielectric strength ranging from1200 volts/mil to 1700 volts/mil. Therefore, a dielectric thickness of0.035 inches thick, provides a safety factor of 2 if the laser operatesat 25 kV. This design yields a lower capacitance; however, the actualeffect of this reduced capacitance on laser operation was discovered tobe negligible, with a surprising increase in the measured geometricirradiation of the electrode gap. Because of the constant diameter,thicker tube wall, integral bushing design, a single piece of materialcan be machined to provide anti-tracking grooves 170. Because of thesingle piece construction, there is no need to use ultra-pure (i.e.,99.9%) polycrystalline translucent aluminum oxide ceramic, althoughApplicants continue to use the ultra-pure material. There is norequirement to perform the difficult surface polishing of tubegeometries in preparation for diffusion bonding to artificially createthe integral relationship between bushing 180 and tube 145. In fact, ithas been determined that high purity is not as important a property asporosity of the material. It has been found that the greater theporosity, the more the dielectric strength is reduced. As a result, acommercial grade ceramic, preferably with purity of at least 99.8% andlow porosity, such as that manufactured by Coors Ceramics Company underthe material No. AD-998E and having a dielectric strength of 300volts/mil may be used. Bushings 180, having anti-tracking grooves 170disposed therein, as previously described, act to prevent high voltagetracking axially along the surface of the tube from the cathode to theground plane 160.

As explained above, Applicants have discovered that a single preionizerworks dramatically better than two preionizers, and as explained abovethe first preferred embodiment places the single preionizer system ofthe electrodes. Applicants have also experimented with the singlepreionizer located downstream and has discovered that at certain blowerspeeds this arrangement produces substantially better pulse energystability than the upstream arrangement on the two tube arrangement.

High Efficiency Chamber

Improvements have been made to the chamber to improve the efficiency ofthe laser. A single piece cathode insulator 276 comprised of alumina,Al₂ O₃ insulates the cathode from the upper chamber structure as shownin FIG. 17. In a prior art design, eight separate insulators were neededto avoid insulator cracking due to thermal expansion stress in theinsulator. This important improvement permitted the head portion of thechamber to be made shorter which significantly reduced the distancebetween the cathode the peaking capacitor. The individual capacitors 54Aforming the peaking capacitor array 82 were moved horizontally in closerto the cathode as compared to the prior art.

Prior art cathodes for commercial lithography lasers were typicallysupported by a cathode support bar 53 as shown in FIG. 3. In thispreferred embodiment, the cathode support bar was eliminated and thecathode 83 was made slightly thicker and mounted directly on the singlepiece insulator 55A. The cathode 83 is connected to the high voltageside 82A of peaking capacitor 82 by 15 feed through rods 83A andconnecting nuts 83B. In the preferred embodiment, a new anode supportbar is substantially more massive than prior art anode support bars andcomprises fins located in the gas flow region. Both of these featuresminimize temperature variations of the anode.

FIG. 20 is a cross-sectional view showing a portion of an upperenclosure member 212 to which is attached the cathode assembly includingan improved feedthrough structure 280, in accordance with the presentinvention. Among other things, feedthrough structure features a singlepiece integrated main insulator 55A including self-contained fifteencylindrical integral feedthrough insulators 288 and polished sealingsurfaces. Upper enclosure member 212 includes fifteen clearance holes232 that are shaped, such that structural alignment is maintainedwithout constraining thermal expansion. Sliding face seals 184, 185 areset into properly dimensioned grooves 234 in the inside surface of upperenclosure 212 and grooves 174 in the top of electrodes 218. These sealsprovide gas-tight integrity without transmitting torque withinfeedthrough structure 280, since they slip on polished surfaces of maininsulator 276.

A cathod 218, made of brass is fastened to an upper enclosure member 212using feedthrough structure 280, such that cathod 218 is positionedabove and aligned longitudinally parallel with an anode assembly 220.Along the centerline of the upper surface of cathode 218 are fifteen ofsubstantially identical evenly spaced blind tapped holes. Concentricwith each of tapped holes is annular groove 174.

Feedthrough structure 280 provides mechanical support, high pulsevoltage electrical contact, and electrical isolation for cathode 83, aswell as sealing against gas leakage around the electrical feedthroughs.Feedthrough structure 280 includes a shaped, longitudinally extendedsingle piece integrated main insulator 55A, generally made of alumina,having a single row of multiple substantially evenly spaced verticalholes concentric through substantially cylindrical integral feedthroughinsulators 288 extending perpendicular from an upper face of singlepiece integrated main insulator 55A in alignment with boind tapped holesof cathod 83. Single piece integrated main insulator 55A is fabricatedusing methods known in the art, e.g., casting and maching “gteen”ceramic material, high-temperature firing, and then finish machining thefired ceramic. Both upper and lower faces of integrated main insulator55A are polished, typically to a surface finish of 16 microinches (0.41microns), providing improved sealing against cathode 218 and upperenclosure member 212. Integral feedthrough insulators 288 each have twoouter concentric grooves 289 as shown in FIG. 17. Feedthrough structure280 also includes multiple (one per each blind tapped hole in cathode 83threaded feedthrough bolts 296, large diameter and small diameter faceseals 184 and 174 respectively comprising tin-plated “C”-seals.Feedthrough structure 280 further includes multiple silicone rubbergaskets 194, properly sized Belleville washers 295, and insulatingceramic “buttercups” 298.

Metal Seals

Applicants have discovered that prior art elastomer seals reacted withfluorine gas to produce contaminants in the laser gas which degradedlaser performance. A preferred embodiment of the present invention usesall metal seals to seal the laser chamber. The preferred metal seals aretin plated inconel 1718 seals. In the case of heat exchanger 11, a metalbellows has been provided which permits thermal expansion andcontraction and allows metal seals to be used to seal cooling waterinlets.

Monel Current Return and Vanes

Applicants have also discovered that elements of stainless steel alsoreact with fluorine to produce contaminants in the laser gas. Therefore,in this preferred embodiment, prior art stainless steel current returnstructures and gas flow vanes have been replaced with monel currentreturns 250 and monel flow vanes 252 and 254.

Acoustic Baffles

Applicants have discovered that a significant cause of distortion of thequality of laser beams produced by narrow-band excimer lasers operatingat 1000 Hz or greater is acoustic shock waves created by the electricdischarge of one pulse which reflects from elements of chamber structureback to the space between the electrodes and distorts the laser beam ofthe next pulse occurring 1.0 millisecond later. An embodiment describedherein (see FIG. 6A) substantially minimizes this effect by providingangled, grooved acoustic baffles 60 and 62 on both sides of the laserchamber. These baffles absorb a portion of the acoustic energy andreflect a portion of the acoustic energy down into the lower region ofthe laser chamber away from the electrodes. In this preferredembodiment, the baffles consist of a machined metal structure withgrooves 0.1 mil wide, 0.3 mil deep spaced at 0.2 mil intervals; a 0.3mil deep groove is shown at 61 in baffle 60 in FIG. 6A. These baffleshave been shown by actual testing to substantially reduce pulse qualitydistortion caused by acoustic shock waves.

Applicants have also discovered that acoustic shock effects can beminimized by reducing streamers in the electric discharge. In fact, in apreferred embodiment of the present invention changes made in thechamber head (discussed above) and the new preionizer designed reducedacoustic shock so that acoustic baffles were not needed.

Fan Improvements

This preferred embodiment of the present invention includes majorimprovements in the prior art gas circulator which has greatly improvedlaser performance. These improvements are in the construction of a brazefree blower blade structure. A new non-symmetrical blade arrangementwhich greatly decreases resonance effects and improved bearings.

Silicon Free Fan Blade Structure

Applicants have discovered that a brazing material commonly used inblower blade construction was the primary source of SiF₆ in the laserchamber. This gas significantly degraded laser performance for KrFlasers but was a total disaster for ArF lasers. Applicants haveidentified four solutions to this problem. First the blade structure wasmachined in segments from a solid block of material (in this casealuminum). Another solution was to cast the blade structure in segments.The segments then are welded together using electron beam welding inwhich no new material is added. It is also feasible to fabricate theblade structure by joining blades to a frame structure but in this casethe joining is by electron beam welding instead of the prior art brazingprocess. The fourth method is to join the blade to a frame structureusing a soldering process using a silicon free solder. Aluminum 6061 isused as the base material for all of the component pieces. These partsare then copper-plated in prelude to the soldering process. With all ofthe parts assembled, the fan is then soldered together using a lowtemperature solder, typically 91% tin (Sn) and 9% Zinc (Zn) in a vacuumfurnace. This solder is chosen due to its lack of silicon and itsability to work with copper plated aluminum. The assembled and solderedfan is then nickel-plated. This method of construction yields anon-silicon fan that is inexpensive to manufacture.

Reducing Resonance Effects

Prior art blower blade structures consisted of a tangential blower with23 longitudinal blades. These blades were mounted symmetrically at thecircumference of the structure. Substantial resonance effects weremeasured both with respect to fan parameters and actual laserperformance. Perturbations in the laser beam were shown to correspond toacoustic waves at 23 times the rotating frequency of the fan. Adverseaffects on bearing performance were also measured corresponding to 23times the fan's rotating frequency.

Improvements in fan structure design call for a non symmetrical bladearrangement such as that shown in FIG. 14A. An alternative as shown inFIG. 14B where the fan blade structure is formed of 16 separate machinedor cart segments with each segment having 23 blades is to rotate eachsegment by 360°/(15×23) or about 1° relative to the adjacent segment.Another improvement which is made relatively easy in the machine or castapproach to fan blade structure fabrication is to form the blades intoair foils as shown at 320 in FIG. 14C. Prior art blades were stamped anda cross section of the two of the stamped blades are shown forcomparison at 314. The direction of rotation is shown at 318 and 330represents the circumference of the blade structure. Whereasconventional blades are uniform in thickness, airfoil blades have a tearshape profile including a rounded leading edge, a thickened midsectionand a tapered trailing edge.

Bearing Improvements

Embodiments of the present invention will be made available with one oftwo alternative bearing improvements over the prior art.

Ceramic Bearings

A preferred embodiment of the present invention includes ceramicbearings. The preferred ceramic bearings are silicon nitride lubricatedwith a synthetic lubricant, preferably perfluoropolyalkylether (PFPE).These bearings provide substantially greater life as compared to priorart excimer laser fan bearings. In addition, neither the bearings northe lubricant are significantly affected by the highly reactive fluorinegas.

Magnetic Bearings

Another preferred embodiment of the present invention comes withmagnetic bearings supporting the fan structure as shown in FIG. 5. Inthis embodiment, the shaft 130 supporting the fan blade structure 146 isin turn supported by an active magnetic bearing system and driven by abrushless DC motor 130 in which the rotor 129 of the motor and therotors 128 of at least two bearings are sealed within the gasenvironment of the laser cavity and the motor stator 140 and the coils126 of the magnetic bearing magnets are located outside the gasenvironment. This preferred bearing design also includes an activemagnetic thrust bearing 124 which also has the coils located outside thegas environment.

Aerodynamic Anode Support Bar

As shown in FIG. 3, prior art gas flow from blower 10 was forced to flowbetween electrodes 6A and 6B by anode support bar 44. However,Applicants have discovered that the prior art designs of support bar 44such as that shown in FIG. 3 produced substantial aerodynamic reactionforces on the blower which were transferred to the blower bearingsresulting in chamber vibration. Applicants suspect that thesevibrational forces are responsible for blower bearing wear and possiblyoccasional bearing failures. Applicant has tested other designs, severalof which are shown in FIGS. 12A-12E, all of which reduced theaerodynamic reaction forces by distributing over a longer time period,the reaction force resulting each time a blade passes close to the edgeof support bar 44. One of Applicants preferred anode support bar designis shown in FIG. 6A at 84A. This design has substantially greater masswhich minimizes anode temperature savings. The total mass of the anodeand the anode support bar is about 3.4 Kg. Also, this design comprisesfins 84B which provides added cooling for the anode. Applicants testshave indicated that both the acoustic baffles and the aerodynamic anodesupport bar tend to reduce slightly the gas flow so that is gas flow islimited, the utilization of these two improvements should involve atrade-off analysis. For these reasons two improvements are shown on FIG.6A and not FIG. 6.

Pulse Power System Functional Description of Four Pulse Power Modules

A preferred pulse power system is manufactured in four separate modulesas indicated in FIGS. 8A and 8B, each of which becomes an important partof the excimer laser system and each of which can be quickly replaced inthe event of a parts failure or in the course of a regular preventativemaintenance program. These modules are designated by Applicants: highvoltage power supply module 20, commutator module 40, compression headmodule 60 and laser chamber module 80.

High Voltage Power Supply Module

High voltage power supply module 20 comprises a 300 volt rectifier 22for converting 208 volt three phase plant power from source 10 to 300volt DC. Inverter 24 converts the output of rectifier 22 to highfrequency 300 volt pulses in the range 100 kHz to 200 kHz. The frequencyand the on period of inverter 24 are controlled by the HV power supplycontrol board 21 in order to provide course regulation of the ultimateoutput pulse energy of the system. The output of inverter 24 is steppedup to about 1200 volts in step-up transformer 26. The output oftransformer 26 is converted to 1200 volts DC by rectifier 28 whichincludes a standard bridge rectifier circuit 30 and a filter capacitor32. DC electrical energy from circuit 30 charges 8.1 μF C_(o) chargingcapacitor 42 in commutator module 40 as directed by HV power supplycontrol board 21 which controls the operation of inverter 24 as shown inFIG. 8A. Set points within HV power supply control board 21 are set bylaser system control board 100.

The reader should note that in this embodiment as shown in FIG. 8A thatpulse energy control for the laser system is provided by power supplymodule 20. The electrical circuits in commutator 40 and compression head60 merely serve to utilize the electrical energy stored on chargingcapacitor 42 by power supply module 20 to form at the rate of 2,000 to4,000 times per second or greater an electrical pulse, to amplify thepulse voltage and to compress in time the duration of the pulse. As anexample of this control, FIG. 8A indicates that processor 102 in controlboard 100 has controlled the power supply to provide precisely 700 voltsto charging capacitor 42 which during the charging cycle is isolatedfrom the down stream circuits by solid state switch 46. The electricalcircuits in commutator 40 and compression head 60 will upon the closureof switch 46 very quickly and automatically convert the electricalenergy stored on capacitor 42 into the precise electrical dischargepulse across electrodes 83 and 84 needed to provide the next laser pulseat the precise energy needed as determined by processor 102 in controlboard 100.

Commutator Module

Commutator module 40 comprises C_(o) charging capacitor 42, which inthis embodiment is a bank of capacitors connected in parallel to providea total capacitance of 8.1 μF. Voltage divider 44 provides a feedbackvoltage signal to the HV power supply control board 21 which is used bycontrol board 21 to limit the charging of capacitor 42 to the voltage(called the “control voltage”) which when formed into an electricalpulse and compressed and amplified in commutator 40 and compression head60 will produce the desired discharge voltage on peaking capacitor 82and across electrodes 83 and 84.

In order to provide electrical pulses in the range of about 3 Joules and16,000 volts at a pulse rate of 2000 Hz pulses per second), about 250microseconds are required for power supply 20 to charge the chargingcapacitor 42 to 800 volts. To achieve faster charging times two or morepower supplies 20 can be stacked. Two stacked power supplies reduce thecharging times to about 150 microseconds. For much faster chargingtimes, a resonance charging system described below can be utilized.Therefore, charging capacitor 42 is fully charged and stable at thedesired voltage when a signal from commutator control board 41 closessolid state switch 44 which initiates the very fast step of convertingthe 3 Joules of electrical energy stored on charging capacitor C_(o)into a 16,000 volt discharge across electrodes 83 and 84. For thisembodiment, solid state switch 46 is a IGBT switch, although otherswitch technologies such as SCRs, GTOs, MCTs, etc. could also be used. A600 nH charging inductor 48 is in series with solid state switch 46 totemporarily limit the current through switch 46 while it closes todischarge the C_(o) charging capacitor 42.

Pulse Generation Stage

The first stage of high voltage pulse power production is the pulsegeneration stage 50. To generate the pulse the charge on chargingcapacitor 42 is switched onto C₁ 8.5 μF capacitor 52 in about 5 μs asshown on FIG. 8F2 by closing IGBT switch 46.

First Stage of Compression

A saturable inductor 54 initially holds off the voltage stored oncapacitor 52 and then becomes saturated allowing the transfer of chargefrom capacitor 52 through 1:23 step up pulse transformer 56 to C_(p−1)capacitor 62 in a transfer time period of about 550 ns for a first stageof compression.

The design of pulse transformer 56 is described below. The pulsetransformer is extremely efficient transforming a 700 volt 17,500 ampere550 ns pulse rate into a 16,100 volt, 760 ampere 550 ns pulse which isstored very temporarily on C_(p−1) capacitor bank 62 in compression headmodule 60.

Compression Head Module

Compression head module 60 further compresses the pulse.

Second Stage of Compression

An L_(p−1) saturable inductor 64 (with about 125 nH saturatedinductance) holds off the voltage on 16.5 nF C_(p−1) capacitor bank 62for approximately 550 ns then allows the charge on C_(p−1) to flow (inabout 100 ns) onto 16.5 nF Cp peaking capacitor 82 located on the top oflaser chamber 80 and which is electrically connected in parallel withelectrodes 83 and 84 and preionizer 56A. This transformation of a 550 nslong pulse into a 100 ns long pulse to charge Cp peaking capacitor 82makes up the second and last stage of compression as indicated at 65 onFIG. 8A.

Laser Chamber Module

About 100 ns after the charge begins flowing onto peaking capacitor 82mounted on top of and as a part of the laser chamber module 80, thevoltage on peaking capacitor 82 has reached about 14,000 volts anddischarge between the electrodes begins. The discharge lasts about 50 nsduring which time lasing occurs within the optical resonance chamber ofthe excimer laser. The optical resonance chamber described in detailbelow is defined by a line narrowing package 86 comprised in thisexample by a 3-prism beam expander, a tuning mirror and an eschellegrating and an output coupler 88. The laser pulse for this laser is anarrow band, 20 to 50 ns, 193 nm pulse of about 5 mJ and the repetitionrate is 1000 pulses per second. The pulses define a laser beam 90 andthe pulses of the beam are monitored by photodiode 92, all as shown inFIG. 8A.

Control of Pulse Energy

The signal from photodiode 94 is transmitted to processor 102 in controlboard 100 and the processor uses this energy signal and preferably otherhistorical pulse energy data (as discussed below in the section entitledPulse Energy Control Algorithm) to set the command voltage for the nextand/or future pulses. In a preferred embodiment in which the laseroperates in a series of short bursts (such as 100 pulse 0.5 secondbursts at 1000 Hz separated by a dead time of about 0.1 second)processor 102 in control board 100 is programmed with a specialalgorithm which uses the most recent pulse energy signal along with theenergy signal of all previous pulses in the burst along with otherhistorical pulse profile data to select a control voltage for thesubsequent pulse so as to minimize pulse-to-pulse energy variations andalso to minimize burst-to-burst energy variations. This calculation isperformed by processor 102 in control board 100 using this algorithmduring a period of about 35 μs. The laser pulses occurs about 5 μsfollowing the T_(o) firing of IGBT switch 46 shown on FIG. 8F3 and about20 μs are required to collect the laser pulse energy data. (The start ofthe firing of switch 46 is called T_(o).) Thus, a new control voltagevalue is thus ready (as shown on FIG. 8F1) about 70 microseconds afterthe firing of IGBT switch 46 for the previous pulse (at 2,000 Hz thefiring period is 500 μs). The features of the energy control algorithmare described below and are described in greater detail in U.S. patentapplication Ser. No. 09/034,870 which is incorporated herein byreference.

Energy Recovery

This preferred embodiment is provided with electronic circuitry whichrecovers excess energy onto charging capacitor 42 from the previouspulse. This circuitry substantially reduces waste energy and virtuallyeliminates after ringing in the laser chamber 80.

The energy recovery circuit 57 is comprised of energy recovery inductor58 and energy recovery diode 59, connected in series across Co chargingcapacitor 42 as shown in FIG. 8B. Because the impedance of the pulsepower system is not exactly matched to that of the chamber and due tothe fact that the chamber impedance varies several orders of magnitudeduring the pulse discharge, a negative going “reflection” is generatedfrom the main pulse which propagates back from the chamber towards thefront end of the pulse generating system. After the excess energy haspropagated back through the compression head 60 and the commutator 40,switch 46 opens up due to the removal of the trigger signal by thecontroller. The energy recovery circuit 57 reverses the polarity of thereflection which has generated a negative voltage on the chargingcapacitor 42 through resonant free wheeling (a half cycle of ringing ofthe L-C circuit made up of the charging capacitor 42 and the energyrecovery inductor 58) as clamped against reversal of current in inductor58 by diode 59. The net result is that substantially all of thereflected energy from the chamber 80 is recovered from each pulse andstored on charging capacitor 42 as a positive charge ready to beutilized for the next pulse. FIG. 8F1, 2 and 3 are time line chartsshowing the charges on capacitor Co, C₁, C_(p−1) and Cp. The charts showthe process of energy recovery on Co.

Magnetic Switch Biasing

In order to completely utilize the full B-H curve swing of the magneticmaterials used in the saturable inductors, a DC bias current is providedsuch that each inductor is reverse saturated at the time a pulse isinitiated by the closing of switch 46.

In the case of the commutator saturable inductors 48 and 54, this isaccomplished by providing a bias current flow of approximately 15Abackwards (compared to the directional normal pulse current flow)through the inductors. This bias current is provided by bias currentsource 120 through isolation inductor LB1. Actual current flow travelsfrom the power supply through the ground connection of the commutator,through the primary winding of the pulse transformer, through saturableinductor 54, through saturable inductor 48, and through isolationinductor LB1 back to the bias current source 120 as indicated by arrowsB1.

In the case of compression head saturable inductor, a bias current B2 ofapproximate 5A is provided from the second bias current source 126through isolation inductor LB2. At the compression head, the currentsplits and the majority B2-1 goes through saturable inductor Lp-1 64 andback through isolation inductor LB3 back to the second bias currentsource 126. A smaller fraction of the current B2-2 travels back throughthe HV cable connecting the compression head 60 and the commutator 40,through the pulse transformer secondary winding to ground, and through abiasing resistor back to the second bias current source 126. This secondsmaller current is used to bias the pulse transformer so that it is alsoreset for the pulsed operation. The mount of current which splits intoeach of the two legs is determined by the resistance in each path and isintentionally adjusted such that each path receives the correct amountof bias current.

Direction of Current Flow

In this embodiment, we refer to the flow of pulse energy through thesystem from a standard three-phase power source 10 to the electrodes andto ground beyond electrode 84 as “forward flow” and this direction asthe forward direction. When we refer to an electrical component such asa saturable inductor as being forward conducting we mean that it isbiased into saturation to conduct “pulse energy” in a direction towardthe electrodes. When it is reverse conducting it is biased intosaturation to conduct energy in a direction away from the electrodestoward the charging capacitor. The actual direction of current flow (orelectron flow) through the system depends on where you are within thesystem. The direction of current flow is now explained to eliminate thisas a possible source of confusion.

By reference to FIGS. 8A and 8B, in this preferred embodiment Cocapacitor 42 is charged to (for example) a positive 700 volts such thatwhen switch 46 is closed current flows from capacitor 42 throughinductor 48 in a direction toward C₁ capacitor 52 (which means thatelectrons are actually flowing in the reverse direction). Similarly, thecurrent flow is from C₁ capacitor 52 through the primary side of pulsetransformer 56 toward ground. Thus, the direction of current and pulseenergy is the same from charging capacitor 42 to pulse transformer 56.As explained below under the section entitled “Pulse Transformer”current flow in both the primary loops and the secondary loop of pulsetransformer 56 is toward ground. The result is that current flow betweenpulse transformer 56 and the electrodes during the initial portion ofthe discharge (which represents the main portion [typically about 80percent] of the discharge) is in the direction away from the electrodestoward transformer 56. Therefore, the direction of electron flow duringthe main discharge is from ground through the secondary of pulsetransformer 56 temporarily onto C_(p−1) capacitor 62 through inductor64, temporarily onto Cp capacitor 82, through inductor 81, throughelectrode 84 (which is referred to as the discharge cathode) through thedischarge plasma, through electrode 83 and back to ground. Thus, betweenpulse transformer 56 and the electrodes 84 and 83 during the maindischarge electrons flow in the same direction as the pulse energy.Immediately following the main portion of the discharge, currents andelectron flow are reversed and the reverse electron flow is from groundup through the grounded electrode 84, though the discharge space betweenthe electrodes to electrode 83 and back through the circuit throughtransformer 56 to ground. The passage of reverse electron flow throughtransformer 56 produces a current in the “primary” loops of transformer56 with electron flow from ground through the “primary” side of pulsetransformer 56 (the same direction as the current flow of the mainpulse) to ultimately charge Co negative as indicated qualitatively inFIG. 8F2. The negative charge on Co is reversed as shown in FIG. 8F2 andexplained above in the section entitled Energy Recovery.

DETAILED DESCRIPTION OF PULSE POWER COMPONENTS Power Supply

A more detailed circuit diagram of the power supply portion of thepreferred embodiment is shown in FIG. 8C. As indicated in FIG. 8C,rectifier 22 is a 6 pulse phase controlled rectifier with a plus 150 vto −150V DC output. Inverter 24 is actually three invertors 24A, 24B and24C. Invertors 24B and 24C are turned off when the voltage on 8.1 μF Cocharging capacitor 42 is 50 volts less than the command voltage andinverter 24A is turned off when the voltage on Co 42 slightly exceedsthe command voltage. This procedure reduces the charge rate near the endof the charge. Step up transformers 26A, 26B and 26C are each rated at 7kw and transform the voltage to 1200 volt AC.

Three bridge rectifier circuits 30A, 30B and 30C are shown. The HV powersupply control board 21 converts a 12 bit digital command to an analogsignal and compares it with a feedback signal 45 from Co voltage monitor44. When the feedback voltage exceeds the command voltage, inverter 24Ais turned off as discussed above, Q2 switch 34 closes to dissipatestored energy within the supply, Q3 isolation switch 36 opens to preventany additional energy leaving the supply and Q1 bleed switch 38 closesto bleed down the voltage on Co 42 until the voltage on Co equals thecommand voltage. At that time Q1 opens.

Commutator and Compression Head

The principal components of commutator 40 and compression head 60 areshown on FIGS. 8A and 8B and are discussed above with regard to theoperation of the system. In this section, we describe details offabrication of the commutator.

Solid State Switch

In this preferred embodiment solid state switch 46 is an P/N CM 1000HA-28H IGBT switch provided by Powerex, Inc. with offices in Youngwood,Pa.

Inductors

Inductors 48, 54 and 64 comprise saturable inductors similar to thosedescribed in U.S. Pat. Nos. 5,448,580 and 5,315,611. A top and sectionview of a preferred saturable inductor design is shown respectively inFIGS. 8G1 and 8G2. In the inductors of this embodiment, flux excludingmetal pieces such as 301, 302, 303 and 304 are added as shown in FIG.8G2 in order to reduce the leakage flux in the inductors. The currentinput to this inductor is a screw connection at 305 to a bus alsoconnected to capacitor 62. The current makes four and one half loopsthrough vertical conductors. From location 305 the current travels downa large diameter conductor in the center labeled 1A, up six smallerconductors on the circumference labeled 1B, down 2A, up 2B, down all ofthe flux excluder elements, up 3B, down 3A, up 4B and down 4A, and thecurrent exits at location 306. Where a pot like housing 64A serves as ahigh voltage current lead. The “lid” 64B of the saturable inductor iscomprised of an electrical insulator material such as teflon. In priorart pulse power systems, oil leakage from oil insulated electricalcomponents has been a problem. In this preferred embodiment, oilinsulated components are limited to the saturable inductors and the oilis contained in the pot-like oil containing metal housing 64A which is,as stated above, the high voltage connection output lead. All sealconnections are located above the oil level to substantially eliminatethe possibility of oil leakage. For example, the lowest seal in inductor64 is shown at 308 in FIG. 8G2. Since the flux excluding metalcomponents are in the middle of the current path through the inductor,the voltage allowing a reduction in the safe hold-off spacing betweenthe flux exclusion metal parts and the metal rods of the other turns.Fins 307 are provided to increase heat removal.

Capacitors

Capacitor banks 42, 52 and 62 are all comprised of banks of commerciallyavailable off-the-shelf capacitors connected in parallel. Thesecapacitors are available from suppliers such as Murata with offices inSmyrna, Ga. Applicants preferred method of connecting the capacitors andinductors is to solder or bolt them to positive and negative terminalson special printed circuit board having heavy nickel coated copper leadsin a manner similar to that described ins U.S. Pat. No. 5,448,580.

Pulse Transformer

Pulse transformer 56 is also similar to the pulse transformer describedin U.S. Pat. Nos. 5,448,580 and 5,313,481; however, the pulsetransformers of the present embodiment has only a single turn in thesecondary winding and 23 separate primary windings. A drawing of pulsetransformer 56 is shown in FIG. 8D. Each of the 23 primary windingscomprise an aluminum spool 56A having two flanges (each with a flat edgewith threaded bolt holes) which are bolted to positive and negativeterminals on printed circuit board 56B as shown along the bottom edge ofFIG. 8D. Insulators 56C separates the positive terminal of each spoolfrom the negative terminal of the adjacent spool. Between the flanges ofthe spool is a hollow cylinder 1{fraction (1/16)} inches long with a0.875 OD with a wall thickness of about {fraction (1/32)} inch. Thespool is wrapped with one inch wide, 0.7 mil thick Metglas™ 2605 S3A anda 0.1 mil thick mylar film until the OD of the insulated Metglas™wrapping is 2.24 inches. A prospective view of a single wrapped spoolforming one primary winding is shown in FIG. 8E.

The secondary of the transformer is a single stainless steel rod mountedwithin a tight fitting insulating tube of electrical glass. The windingis in four sections as shown in FIG. 8D. The stainless steel secondaryshown as 56D in FIG. 8D is grounded to a ground lead on printed circuitboard 56B at 56E and the high voltage terminal is shown at 56F. Asindicated above, a 700 volt pulse between the + and − terminals of theprimary windings will produce a minus 16,100 volt pulse at terminal 56Fon the secondary side for a 1 to 23 voltage transformation. This designprovides very low leakage inductance permitting extremely fast outputrise time.

Laser Chamber Pulse Power Components

The Cp capacitor 82 is comprised of a bank of twenty-eight 0.59 nfcapacitors mounted on top of the laser chamber pressure vessel. Theelectrodes 83 and 84 are each solid brass bars about 28 inches longwhich are separated by about 0.5 to 1.0 inch. In this embodiment, thetop electrode 83 is the cathode and the bottom electrode 84 is connectedto ground as indicated in FIG. 8A.

Compression Head Mounting

This preferred embodiment of the present invention includes acompression head mounting technique shown in FIGS. 8H1 and 8H2. FIG. 8H1is a side section view of the laser system showing the location of thecompressor head module 60 in relation to electrodes 83 and 84. Thistechnique was designed to minimize the impedance associated with thecompression lead chamber connection and at the same time facilitatesquick replacement of the compression head. As shown in FIGS. 8H1 and 8H2the ground connection is made with an approximately 28 inch long slottab connection along the back side of the compression head as shown at81A in FIG. 8H1 and 81B in FIG. 8H2. The bottom of the slot tab isfitted with flexible finger stock 81C. A preferred finger stock materialis sold under the trade name Multilam®.

The high voltage connection is made between a six-inch diameter smoothbottom of saturable inductor 64 and a mating array of flexible fingerstock at 89 in FIG. 8H1. As above, a preferred finger stock material isMultilam®. This arrangement permits the replacement of the compressionhead module for repair or preventative maintenance in about fiveminutes.

Resonant Charging

In a preferred embodiment of the present invention, the power supplymodule described for the first preferred embodiment which utilizes tworectifiers, an inverter and a transformer as shown in FIGS. 8A and 8B;is replaced by an off-the-shelf power supply and a resonance chargingcircuit. This latter approach provides much faster charging of thecharging capacitor.

First Resonant Charger

An electrical circuit showing this preferred embodiment is shown in FIG.16. In this case, a standard capacitor charging power supply 200 havinga 480 VAC/40 amp input and a 1200 VDC 50 amp output is used. Such powersupplies are available from suppliers such as Ecgar, Maxwell, Kaiser andAle. This power supply continuously charges a 325 μF capacitor 202 tothe voltage level commanded by the control board 204. The control board204 also commands IGBT switch 206 closed and open to transfer energyfrom capacitor 202 to capacitor 42. Inductor 208 sets up the transfertime constant in conjunction with capacitor 202 and 42. Control board202 receives a voltage feedback 212 that is proportional to the voltageon capacitor 42 and a current feedback 214 that is proportional to thecurrent flowing through inductor 208. From these two feedback signalscontrol board 204 can calculate in real time the final voltage oncapacitor 42 should IGBT switch 206 open at that instant of time.Therefore with a command voltage 210 fed into control board 204 aprecise calculation can be made of the stored energy within capacitor 42and inductor 208 to compare to the required charge voltage commanded210. From this calculation, the control board 204 will determine theexact time in the charge cycle to open IGBT switch 206.

After IGBT switch 206 opens the energy stored in the magnetic field ofinductor 208 will transfer to capacitor 42 through the diode path 216.The accuracy of the real time energy calculation will determine theamount of fluctuation dither that will exist on the final voltage oncapacitor 42. Due to the extreme charge rate of this system, too muchdither will exist to meet a desired systems regulation need of ±0.05%.Therefore a bleed down circuit is included in this embodiment.

Bleed down circuit 216 will be commanded closed by the control board 204when current flowing through inductor 208 stops. The time constant ofcapacitor 42 and resistor 220 should be sufficiently fast to bleed downcapacitor 42 to the command voltage 210 without being an appreciableamount of the total charge cycle.

Second Resonant Charger

A second resonant charger system is shown in FIG. 16B. This circuit issimilar to the one shown in FIG. 16A. The principal circuit elementsare:

I1—A three-phase power supply 300 with a constant DC current output.

C-1—A source capacitor 302 that is an order of magnitude or more largerthan the existing C0 capacitor 42.

Q1, Q2, and Q3—Switches to control current flow for charging andmaintaining a regulated voltage on C0.

D1, D2, and D3—Provides current single direction flow.

R1, and R2—Provides voltage feedback to the control circuitry.

R3—Allows for rapid discharge of the voltage on C0 in the event of asmall over charge.

L1—Resonant inductor between C-1 capacitor 302 and C0 capacitor 4-2 tolimit current flow and setup charge transfer timing.

Control Board 304—Commands Q1, Q2, and Q3 open and closed based uponcircuit feedback parameters.

The power supply is a fixed 700-volt power supply. The power supply isattached directly to C-1 eliminating the need for voltage feedback tothe supply. When the supply is enabled it turns on and regulates aconstant current output until the voltage on C-1 capacitor 302 is 700volts, and then it turns off.

The performance of the system is independent of the voltage regulationon C-1 therefore only the most basic control loop is necessary in thepower supply. Secondly the supply will be adding energy into the systemwhenever the voltage on C-1 falls below the 700-volt setting. Thisallows the power supply the entire time between laser pulse, (and evenduring laser pulses), to replenish energy transferred from C-1 to C0.This further reduces the power supply peak current requirements over thepulse power system described above. The power supply is only given apercentage of the total time between laser pulses to charge C0 to adesired voltage. The combination of requiring a supply with the mostbasic control loop, and minimizing the peak current rating of the supplyto the average power requirements of the system reduces the power supplycost an estimated 50%. Additionally this preferred design providesvendor flexibility since constant current, fixed output voltage powersupplies are readily available from multiple sources.

In order to utilize the most basic power supply available on the market,additional circuitry is added that is estimated at a small fraction ofthe total power supply savings. These additional components are outlinedabove with the exception of C0 which is a part of the commutator. Anexample of operation is as follows:

Prior to the need for a laser pulse the voltage on C-1 would be chargedto 700 volts, switches Q1, and Q3 would be open, and Q2 would be closed.Upon command from the laser, Q1 would close and Q2 would open. At thistime current would flow from C-1 to C0 through the charge inductor L1.On the control board would be a calculator that is evaluating thevoltage on C0, and the current flowing in L1 relative to a commandvoltage set point from the laser. Q1 will open when the voltage on C0plus the equivalent energy stored in inductor L1 equals the desiredcommand voltage plus 100 volts. The calculation is:

V _(f) =[V _(C0s) ²+((L ₁ *I _(11s) ²)/C ₀)]^(0.5)+100  (1)

Where:

V_(f)=The voltage on C0 after Q1 opens and the current in L1 goes tozero.

V_(C0s)=The voltage on C₀ when Q1 opens.

I_(L1s)=The current flowing through L₁ when Q1 opens.

The reason for adding 100 volts to the calculation is to ensure currentis still flowing in L1 at the time the voltage on C0 equals the commandvoltage.

After Q1 opens the energy stored in L1 starts transferring to C0 throughD2 until the voltage on C0 equals the command voltage. At this time Q2closes and current stops flowing to C0 and is directed through D3 toproduce a “flywheel current loop.” A window comparitor is used tocontrol the on off switching of Q2. Should the voltage on C0 drop belowthe command voltage less 0.5 volts then Q2 will open and charge currentheld in L1 will flow to C0 until the voltage on C0 equals the commandvoltage plus 0.5 volts.

Should for any reason the voltage on C0 exceed the command voltage plus0.6 volts then Q3 will closed until the voltage on C0 drops to thecommand voltage plus 0.6 volts. This ensures the voltage regulation hasa 0.6 volt variation worst case at the time the laser pulses.

With the above circuit, and a starting charge voltage on C-1 of 700volts, the maximum first pulse voltage on C0 will be 1400 volts. Forthis embodiment, the maximum operating voltage will be 1200 volts. Thisallows the system to hold up to 200 volts of charge in L1 for reserve.This reserve can be incrementally pulsed onto C0 as needed due toleakage losses during the time after C0 equals the command voltage andthe laser discharging C0.

Faster Pulse Rise Time

FIG. 8I shows a modification to the pulse power system which was made byApplicants and their fellow workers to increase somewhat the pulse risetime on C_(p) capacitor 82 and to improve efficiency of the system. Thisconfiguration adds a compression stage. The capacitance values ofC_(p−2) and C_(p−1) are both 16 nF and the saturated inductance ofL_(p−2) is about 10 nH. The unsaturated inductance is very much larger(in the range of 10's of μH's). Test results of this modification areshown in FIG. 8I(1).

Resonance Cavity

In this preferred embodiment of the present invention the reflectivityof the output coupler has been approximately tripled from about 10%which was typical of prior art narrow-band excimer lasers to about 30%.This is to provide more feedback within the laser cavity to reduce thelaser bandwidth and to improve laser efficiency.

Line Narrowing Module Improved Gratings

Applicants have developed a grating highly resistant to photon damage. Apreferred method for fabricating this grating is described in detail inU.S. patent application Ser. No. 08/939,611, filed Sep. 29, 1997, whichis incorporated herein by reference. The first steps in the fabricationof the gratings are similar to the well-known technique of makingreplica gratings from master or submaster gratings. This techniqueproduces a grating having an epoxy substrate with a thin aluminumreflective coating, which may be cracked or has relatively thick grainboundaries containing oxides and hydroxides of aluminum and typically isalso naturally coated with an aluminum oxide film. The grating issubsequently recoated in a vacuum chamber with a thin, pure, densealuminum overcoat and then also in the vacuum the aluminum overcoat iscoated with a thin film of MgF₂. The grating is especially suited foruse for wavelength selection in an ArF laser operating producing anultraviolet laser beam at a wavelength of about 193 nm. The oxygen freealuminum overcoat prevents the ultraviolet light from causing damage bystimulating chemical reactions in grating materials under the aluminumgrating surface or in the aluminum oxide film. The MgF2 additionallyprevents oxidation on the surface of the aluminum overcoat. Across-sectional views of the grating are shown in FIGS. 15A and 15B.

Calcium Fluoride Beam Expanding Prisms

The increase in the reflection of the output coupler and the potentialincrease in the repetition rate had the effect of substantiallyincreasing the light passing through the line-narrowing module. Theadditional heat generated by absorption of this additional illuminationcauses thermal distortion in prior art beam expanding fused silicaprisms. To solve this problem prior art fused silica prisms werereplaced with calcium fluoride prisms. The three beam expanding CaFprisms 437A, B and C are shown in FIG. 15A. Calcium fluoride has higherthermal conductivity and can handle the additional energy withoutunacceptable distortion.

Thermal lensing in CaF prisms is substantially lower than that for fusedsilica for two reasons. CaF prisms have thermal conductivity about 10times that of fused silica and absorption at UV wavelength about onehalf that of fused silica. The advantages of CaF over fused silica havefor some time been recognized by stepper makers but Applicants were tothe best of their knowledge the first to apply the new high quality CaFtechnology to and the production use of CaF prism beam expanders.

Prism 437A is a 1-inch prism mounted with an angle of incidence of 74.0degrees, prism 437B is a 1-inch prism mounted at 74.0 degrees, and prism437C is a two inch prism mounted at 74.0 degrees. The magnification ofthe beam expansion system system is 21 and the dispersion of the linenarrowing system is 1.09 mr/pm.

Improved Wavemeter ArF Natural Spectrum

FIG. 10A shows the approximate natural broadband spectrum of a highpulse rate ArF excimer laser. As shown in FIG. 10A, the FWHM bandwidthis about 472 pm. This particular laser may be operated at a rate of upto 1000 Hz, and the typical pulse energy is about 25 mJ per pulse whenoperated broadband. This broadband spectrum is generally not useful forintegrated circuit lithography which typically requires bandwidths ofless than 1.0 pm.

ArF Narrowband Spectrum

The laser may be narrow banded, using well known prior art techniques.Narrowbanding produces an output spectrum such as that shown in FIG.10B. In this case the FWHM bandwidth is greatly reduced (by a factor ofalmost 800) to about 0.6 pm, and the pulse energy is reduced (by afactor of about 5) to about 5 mJ. As a result, the intensity of thepulse at the desired narrow band is very greatly increased as indicatedin FIG. 10B.

As shown in FIG. 10D, the laser 30 may be tuned to operate at anywavelength within the ArF broadband spectrum using tuning mirror 36 inline-narrowing module 31. In a preferred embodiment, the laser is tunedby pivoting mirror 36 with stepper motor 39 so as to slightly change theangle at which the laser beam (expanded by prisms 37A, 37B and 37C) isincident on grating 38. The relationship between wavelength and mirrorposition as measured by steps of stepper motor 39 is shown in FIG. 10Cwhere one full step of the stepper motor produces a change of about 0.15pm in the nominal narrowband output center wavelength. The stepper motorscan of a few millimeters is sufficient to scan the output wavelength oflaser 30 throughout 500 pm tuning range from about 193,100 pm to about193,600 pm. Note that the relationship between mirror position andwavelength is not exactly linear, but in the narrow tuning range of thislaser, a linear relationship can be assumed and in this preferredembodiment linearity is not required.

Measuring Beam Parameters

FIG. 10 shows the layouts of a preferred wavemeter unit 120A an absolutewavelength reference calibration unit 190, and a wavemeter processor197.

The optical equipment in these units measure pulse energy, wavelengthand bandwidth. These measurements are used with feedback circuits tomaintain pulse energy and wavelength within desired limits. Theequipment calibrates itself by reference to an atomic reference sourceon the command from the laser system control processor.

As shown in FIG. 10, the output beam from output coupler 32 (as shown inFIG. 10D) intersects partially reflecting mirror 170, which passes about95.5% of the beam energy as output beam 33 and reflects about 4.5% forpulse energy, wavelength and bandwidth measurement.

Pulse Energy

About 4% of the reflected beam is reflected by mirror 171 to energydetector 172 which comprises a very fast photo diode 69 which is able tomeasure the energy of individual pulses occurring at the rate of 3000per second. The pulse energy is about 3.32 mJ, and the output ofdetector 69 is fed to a computer controller which uses a specialalgorithm to adjust the laser charging voltage to precisely control thepulse energy of future pulses based on stored pulse energy data in orderto limit the variation of the energy of individual pulses and theintegrated energy of bursts of pulses.

Coarse Wavelength Measurement

About 4% of the beam which passes through mirror 171 is reflected bymirror 173 through slit 177 to mirror 174, to mirror 175, back to mirror174 and onto echelle grating 176. The beam is collimated by lens 178having a focal length of 458.4 mm. Light reflected from grating 176passes back through lens 178, is reflected again from mirrors 174, 175and 174 again, and then is reflected from mirror 179 and focused ontothe left side of 1024-pixel linear photo diode array 180. The spatialposition of the beam on the photo diode array is a coarse measure of therelative nominal wavelength of the output beam.

Linear Photo Diode Array

Linear Photo diode array 180 is depicted in greater detail in FIG. 10E.The array is an integrated circuit chip comprising 1024 separate photodiode integrated circuits and an associated sample and hold readoutcircuit. The photo diodes are on a 25 micrometer pitch for a totallength of 25.6 mm (about one inch). Each photo diode is 500 micrometerlong. Photo diode arrays such as this are available from severalsources. A preferred supplier is Hamamatsu. In our preferred embodiment,we use a Model S3903-1024 which can be read at the rate of 2×10⁶pixels/sec on a FIFO basis in which complete 1024 pixel scans can beread at rates of 2000 Hz or greater.

Calculation of Coarse Wavelength

The coarse wavelength optics in wavemeter module 120 produces arectangular image of about 0.25 mm×3 mm on the left side of photo diodearray 180. The ten or eleven illuminated photo diodes will generatesignals in proportion to the intensity of the illumination received andthe signals are read and digitized by a processor in wavemetercontroller 197. Using this information and an interpolation algorithmcontroller 197 calculates the center position of the image.

This position (measured in pixels) is converted into a coarse wavelengthvalue using two calibration coefficients and assuming a linearrelationship between position and wavelength. These calibrationcoefficients are determined by reference to an atomic wavelengthreference source as described below. For example, the relationshipbetween image position and wavelength might be the following algorithm:

λ=(2.3 pm/pixel)P+191,625 pm

where P=coarse image central positions

Fine Wavelength Measurement

About 95% of the beam which passes through mirror 173 as shown in FIG.10 is reflected off mirror 182 through lens 183 onto a diffuser at theinput to etalon assembly 184. The beam exiting etalon 184 is focused bya 458.4 mm focal length lens in the etalon assembly and producesinterference fringes on the middle and right side of linear photo diodearray 180 after being reflected off two mirrors as shown in FIG. 10.

The spectrometer must measure wavelength and bandwidth substantially inreal time. Because the laser repetition rate may be 1000 H_(Z) orhigher, it is necessary to use algorithms which are accurate but notcomputationally intensive in order to achieve the desired performancewith economical and compact processing electronics. Calculationalalgorithm therefore preferably should use integer as opposed to floatingpoint math, and mathematical operations should preferably be computationefficient (no use of square root, sine, log, etc.).

The specific details of a preferred algorithm used in this preferredembodiment will now be described. FIG. 11B is a curve with 5 peaks asshown which represents a typical etalon fringe signal as measured bylinear photo diode array 180. The central peak is drawn lower in heightthan the others. As different wavelengths of light enter the etalon, thecentral peak will rise and fall, sometimes going to zero. This aspectrenders the central peak unsuitable for the wavelength measurements. Theother peaks will move toward or away from the central peak in responseto changes in wavelength, so the position of these peaks can be used todetermine the wavelength, while their width measures the bandwidth ofthe laser. Two regions, each labeled data window, are shown in FIG. 11B.The data windows are located so that the fringe nearest the central peakis normally used for the analysis. However, when the wavelength changesto move the fringe too close to the central peak (which will causedistortion and resulting errors), the first peak is outside the window,but the second closest peak will be inside the window, and the softwarecauses the processor in control module 197 to use the second peak.Conversely, when the wavelength shifts to move the current peak outsidethe data window away from the central peak the software will jump to aninner fringe within the data window. The data windows are also depictedon FIG. 10F.

The steps involved are as follows:

1. After a laser shot, the photo diode array output is electronicallyread out and digitized. Data points are separated by an intervalphysically determined by the spacing of the photo diode array elements,in this case 25 micrometer pitch.

2. The digital data is searched to find the peak intensity value in thedata window. The previous peak location is used as a starting point.Small regions are searched left and right of the starting point. Thesearch region is extended by small intervals left and right until a peakis found. If the peak is outside the data window, the search willautomatically continue until the other peak is found.

3. Based on the intensity of the peak, a 50% level is computed as shownin FIG. 11A. The 0% level is measured periodically between the pulses.Based on the computed 50% level, points are examined right and left ofthe peak until the data points which border the 50% level are found. Alinear interpolation is computed between pairs of points, which borderthe 50% level to find the left and right half-maximum positions, labeledA, and B in FIG. 11A. These positions are computed to a fraction of apixel such as {fraction (1/16)}, using an integer data format.

4. Steps 2 and 3 are duplicated for the two data windows, giving a totalof four interpolated 50% positions. As indicated FIG. 11B, two diametersare computed. D1 is the inner fringe diameter while D2 is the outerfringe diameter.

5. An approximation to the wavelength is determined by the coarsewavelength circuit, as described in the preceding section “CoarseWavelength Measurement.”

Fine Wavelength Calculation

The inner and outer fringe diameters D1 and D2 (in units of pixels) areeach converted to wavelength by the following equations:

λ=λ₀ +Cd(D ² −D ₀ ²)+N·FSR

where λ=wavelength corresponding to diameter D

λ₀=calibration wavelength

D₀=diameter corresponding to wavelength λ₀

Cd=calibration constant dependant on the optical design

FSR=free spectral range of the etalon

N=integer,=0, ±1, ±2, ±3 . . .

The values λ₀, K₁, FSR, and D₀ are determined and stored at the time ofcalibration. The value for N is chosen such that:

|λ−λ_(c)|≦½ FSR

where λ_(c)=coarse wavelength determination.

For example, in a preferred embodiment, we select a reference wavelengthλ₀=193,436.9 pm (corresponding to an absorption line of a platinumhollow cathode lamp). At this wavelength, the fringe diameter D₀ mightbe found to be 300 pixels. Cd is a constant which can either be directlymeasured or calculated from the optical design. In our preferredembodiment, Cd=−9.25×10⁻⁵ pm/pixel². Thus, for example, with the laseroperating at a different wavelength, the fringe diameter may be measuredto be 405 pixels. The possible wavelengths computed by equation (1) are:

λ=193,436.9 pm−9.25×10⁻⁵ pm/pixel²[(405)²−(300)² ]+N·FSR=193,443,7+N·FSR

If the free spectral range FSR=20 pm, then the possible values for λinclude:

193,403.7 pm N=−2

193,423.7 N=−1

193,443.7 N=0

193,463.7 N=+1

193,483.7 N=+2

If the coarse wavelength is determined to be λ_(c)=193,401.9, forexample, then the processor will select the value λ=193,403.7 pm (N=−2)as the solution in the closest agreement with λ_(c).

The inner and outer fringe diameters D₁ and D₂as shown in FIG. 9B areeach converted into wavelengths λ₁ and λ₂, respectively. The final valuewhich is reported for the laser wavelength is the average of these twocalculations:$\lambda = \left( \frac{\lambda_{1} + \lambda_{2}}{2} \right)$

Bandwidth Calculation

The bandwidth of the laser is computed as (λ₂−λ₁)/2. A fixed correctionfactor is applied to account for the intrinsic width of the etalon peakadding to the true laser bandwidth. Mathematically, a deconvolutionalgorithm is the formalism for removing the etalon intrinsic width fromthe measured width, but this would be far too computation-intensive, soa fixed correction Δλε is subtracted, which provides sufficientaccuracy. Therefore, the bandwidth is:${{\Delta \quad \lambda} = {\left( \frac{D_{2} - D_{1}}{2} \right) - {\Delta \quad \lambda \quad \varepsilon}}}\quad$

Δλε depends on both the etalon specifications and the true laserbandwidth. It typically lies in the range of 0.1-1 pm for theapplication described here.

Calibration with Atomic Reference Source

In this preferred embodiment, wavemeter 120 is calibrated with theoptical equipment shown in atomic wave reference unit 190 as shown inFIG. 10.

The approximately 5% portion of the beam passing through mirror 182 isreflected from mirror 186 through lens 188 and into atomic wavelengthreference unit 190. The light passes through diffuser 191, reflects offmirror 192 and is focused by lens 193 to a focal point in the center ofvapor cell 194, and is focused again by lens 195 onto photo diode 196.Calibration is accomplished by scanning the wavelength of the laser(with tuning mirror 36 as shown in FIG. 10D) while keeping the outputenergy of the laser constant, as monitored and controlled by energydetector 69. In this preferred embodiment, the scanning and calibrationsequence is automated and programmed into the control electronics forthe laser. The wavelength range of the scan is chosen so as to includean absorption wavelength of the platinum vapor cell 194. For example, inthis preferred embodiment, the strong absorption at 193,436.9 pm isused, and the laser is programmed to scan from about 193,434 pm to193,440 pm. When the laser wavelength coincides with the absorptionwavelength, a substantial reduction in signal (10-50%) is seen by thephoto diode 196. During the scan, two corresponding sets of data aretaken, the signal from the photo diode 196, and the wavelength asmeasured by the wavemeter 120. A representative set of data is shown inFIG. 10J, where the signal from the photo diode 196 is plotted againstthe wavelength as reported by the wavemeter 120. The processor analyzesthe photo diode data and determines the apparent wavelength λ₁ whichcorresponds to the center of the absorption dip. Since the truewavelength λ₀ of the atomic absorption reference is known withprecision, the calibration error (λ₁−λ₀) can be calculated. This erroris then used to automatically correct the calibration constants used byboth the coarse and fine wavelength algorithms.

Improved Etalon

Conventional etalon mounting schemes typically employ an elastomer tomount the optical elements to the surrounding structure, to constrainthe position of the elements but minimize forces applied to theelements. A compound commonly used for this is room-temperaturevulcanizing silicone (RTV). However, various organic vapors emitted fromthese elastomers can deposit onto the optical surfaces, degrading theirperformance. In order to prolong etalon performance lifetime, it isdesirable to mount the etalon in a sealed enclosure that does notcontain any elastomer compounds.

A preferred embodiment includes an improved etalon assembly shown at 184in FIG. 10. In this etalon assembly is shown in detail in FIGS. 10G1 and10G2, the fused silica etalon 79 itself is comprised of a top plate 80having a flange 81 and a lower plate 82, both plates being comprised ofpremium grade fused silica. The etalon is designed to produce fringeshaving free spectral range of 20.00 pm at 193.35 nm when surrounded bygas with an index of refraction of 1.0003 and a finesse equal to orgreater than 25. Three fused silica spacers 83 with ultra low thermalexpansion separate the plates and are 934 micrometer±1 micrometer thick.These hold the etalon together by optical contact using a technique wellknown in the optics manufacturing art. The reflectance of the insidesurfaces of the etalon are each about 88 percent and the outsidesurfaces are anti-reflection coated. The transmission of the etalon isabout 50 percent.

The etalon 79 is held in place in aluminum housing 84 only by gravityand three low force springs 86 pressing the flange against three padsnot shown but positioned on 120 degree centers under the bottom edge offlange 81 at the radial location indicated by leader 85. A clearance ofonly 0.004 inch along the top edge of flange 81 at 87 assures that theetalon will remain approximately in its proper position. This closetolerance fit also ensures that if any shock or impulse is transferredto the etalon system through the mounting, the relative velocitiesbetween the optical components and the housing contact points will bekept to a minimum. Other optical components of etalon assembly 184include diffuser 88, window 89 and focusing lens 90 having a focallength of 458.4 mm.

The diffuser 88 may be a standard prior art diffuser commonly usedup-stream of an etalon to produce a great variety of incident anglesneeded for the proper operation of the etalon. A problem with prior artdiffusers is that about 90 percent of the light passing through thediffuser is not at a useful angle and consequently is not focused on thephoto diode array. This wasted light, however, adds to the heating ofthe optical system and can contribute to degradation of opticalsurfaces. In an alternative embodiment a diffractive lens array is usedas the diffuser 88. In this case a pattern is produced in thediffractive lens array which scatters the light thoroughly but onlywithin an angle of about 5 degrees. The result is that about 90 percentof the light falling on the etalon is incident at useful angles and amuch greater portion of the light incident on the etalon is ultimatelydetected by the photo diode array. The result is the light incident onthe etalon can be greatly reduced which greatly increases opticalcomponent life. Applicants estimate that the incident light can bereduced to less than 10% of prior art values with equivalent light onthe photo diode array.

Platinum Vapor Cell

Details of platinum vapor cell 194 are described by reference to FIG.10K. This cell is a modified series L2783 hollow cathode lamp tubesimilar to the one described in U.S. Pat. No. 5,450,202. A glassenvelope with UV-transmitting windows 314 and 316 contains neutral gas,neon. The major difference is that the hollow cathode of the cell inthis preferred embodiment comprises a very thin platinum “hollow T”shaped sleeve which covers the surface of the cathode 320 facing anode318 and the inside surface of hollow cathode 320. A DC source of about150 volts energizes the cell creating a plasma containing platinum ionswhich are generally contained within the hollow cathode as a vaporousmaterial as shown in FIG. 10K.

OTHER PREFERRED EMBODIMENTS

In another preferred embodiment of this invention, focusing element 193may be used with a suitable aperture to allow a collimated portion ofthe beam to pass through the cathode 198, through optical element 195and onto detector 196. In this embodiment, measures must be taken toavoid optical interference effects due to reflections from parallelsurfaces of the windows in cell 194. Such measures might includeproviding a small wedge angle between the inner and outer windowsurfaces on both the entrance and exit windows of cell 194.

Since the platinum vapor provides two distinctive absorption lineswithin the tuning range of the ArF laser, both lines are available ifneeded to improve the accuracy of the calibration. Procedures could beestablished to use both lines on each calibration. But preferably thesecond line could be checked only occasionally if the occasional checksshow the calibration at one line calibrates the laser accurately at thesecond line.

Other Techniques for Tuning the Laser

Tuning of laser 30 of FIG. 10D to a desired wavelength may be performedin various well known ways, which may be mechanically, optically,although tuning the laser optically, as shown in FIG. 10D, is preferred.Any of these known mechanisms for tuning a laser in response to acontrol signal may constitute the wavelength adjustment mechanism.

Although a specific structure is shown for detecting platinum absorptionlines other suitable embodiments may be employed. These may beimplemented using different optical setups, as would be understood bythose skilled in the art after reviewing this disclosure.

Laser Component Cooling

Preferred embodiments of the present invention which is especiallyuseful for operation at repetition rates in excess of 1000 Hz, includesa unique cooling technique shown in FIG. 17 for cooling an excimerlaser.

Components of the laser are contained in enclosure 240 which ismaintained on the inside at a slight vacuum produced by a blower mountedin a vent as shown at 224 in FIGS. 17 and 4A. The cabinet comprisesfiltered intake port 241 near the top of the cabinet and a few smallleakage sources, such as around gasketed doors, so that the flow of roomair through the laser enclosure is about 200 ft³/min which is not nearlysufficient to remove the heat produced by heat producing components ofthe laser.

The very great majority (roughly 90 percent) of the waste heat producedby the laser (roughly 12 kw at 100% duty factor) is removed by a chilledwater system as shown in FIG. 17.

In this embodiment the major heat sources in the laser are the highvoltage supply 20, the commutator 40, the compression head 60 and thelaser chamber 80. For the chamber a water cooled heat exchanger islocated inside the chamber and heat is transferred from circulatinglaser gas to the heat exchanger to the cooling water. Another heatexchanger (not shown) is mounted on an outside surface of the chamber.For the rest of the major heat producing components cooling water ispiped to the location of the component and one or more fans force airthrough a water-to-air heat exchanger onto the component as shown inFIG. 17. For the compression head the circulation is contained as shown,but for the HVPS and the commutator the circulation is onto thecomponent then through other portions of the enclosure to also coolother components before being recirculated back to the heat exchangers.

Dividing pans 242 and 243 guide the general ventilation air from filter241 through a path shown by open headed arrows 244 to vent 224.

This cooling system contains no ducts and except for a water linefeeding the heat exchangers inside of and attached to the laser chamberthere is no water line connection to any laser component. Since allcomponents (other than the laser chamber) are cooled by air blown aboutinside the enclosure, there are no cooling connections to make a breakwhen installing and replacing components. Also, the lack of need forducting greatly increases useable components and working space insidethe enclosure.

Laser Performance with Low Pulse Energy and High Repetition Rate

Applicants have tested a prototype laser configured as a preferredembodiment of the present invention. The results of this testing arecompared in FIGS. 17A through 19.

FIGS. 17A, B and C show typical performance data on a lithography laseroperated at 2,000 Hz and 5 mJ. This system was considered by Applicantsto be a baseline for measuring the performance of their invention. Thedata shows a dose stability of 9.31%, an FWHM bandwidth of 0.4 pm, anintegral 95 bandwidth of 1.1 pm and a TIS pulse duration of 30 ns.

Applicants use of an italon output coupler matched to the line narrowingunit permitted reduction of the bandwidth to 0.3 pm (FWHM) and 0.66(I95) as shown in FIG. 18.

FIG. 19 shows a comparison of dose stability for the lower pulse energyhigher pulse rate configuration and the baseline line configuration.

Although this very narrow-band ArF excimer laser has been described withreference to a particular embodiment, it is to be appreciated thatvarious adaptations and modifications may be made. For example, manyalternative embodiments are discussed in the patent applications listedin the first sentence of this specification, all of which have beenincorporated herein by reference. An etalon output coupler could be usedto provide additional line narrowing. The invention is to be limitedonly by the appended claims.

What is claimed is:
 1. A very narrow band reliable modular productionquality high repetition rate ArF excimer laser for producing a narrowband pulsed laser beam at repetition rates of at least about 3000 Hz,said laser comprising: A. a quickly replaceable laser chamber modulecomprising a laser chamber comprising: 1) two elongated electrodes; 2) alaser gas comprised of a) argon, b) fluorine, and c) a neon gas; 3) agas circulator for circulating said gas between said electrodes atspeeds of at least two cm/millisecond B. a modular pulse power systemcomprised of at least one quickly replaceable module, said system beingcomprised of a power supply and pulse compression and amplificationcircuits and pulse power controls for producing high voltage electricalpulses of at least 14,000 volts across said electrodes at rates of atleast about 3000 Hz; C. a quickly replaceable line narrowing module forcontrolling wavelengths of said laser beams to less than that 0.4 pm,FWHM; and D. a laser pulse energy control system for controlling thevoltage provided by said pulse power system, said control systemcomprising a laser pulse energy monitor and a computer processorprogrammed with an algorithm for calculating, based on historical pulseenergy data, electrical pulses needed to produce laser pulses havingpulse energies within a desired range of energies of less than
 04. mJ.2. A laser as in claim 1 wherein said chamber, said pulse power system,said line narrowing system, said energy control system and substantiallyall electrical, optical and mechanical components of said laser arecontained in quickly replaceable modules.
 3. A laser as in claim 1wherein said chamber and said gas circulator define a gas flow path andan upstream direction and said laser also comprises a single preionizertube located upstream of said electrodes.
 4. A laser as in claim 1wherein each of said electrodes define an electrode length and saidsingle preionizer tube is comprised of a grounded electricallyconducting rod positioned along the axis of an Al₂O₃ hollow cylindricaltube having a length longer than said electrode length.
 5. A laser as inclaim 1 wherein said line narrowing system comprises at least three beamexpanding prisms, at least one of which prisms is comprised of calciumfluoride, a tuning mirror and a grating.
 6. A laser as in claim 1wherein all of said at least three prisms are comprised of calciumfluoride.
 7. A laser as in claim 1 wherein said two elongated electrodesdefine a cathode and an anode and said anode comprises cooling fins. 8.A laser as in claim 1 wherein said laser chamber defines a chamberstructure and wherein said two elongated electrodes define a cathode andan anode and said cathode is insulated from said chamber structure by asingle piece insulator comprised of Al₂O₃ which is attached to a portionof said chamber structure.
 9. A laser as in claim 8 wherein said cathodeis mounted directly on said single piece insulator.
 10. A laser as inclaim 1 wherein all seals exposed to said laser gas are metal seals. 11.A laser as in claim 1 and further comprising acoustic baffles.
 12. Alaser as in claim 1 wherein said power supply comprises a rectifier forconverting AC power to DC power, an inverter for converting the DC powerto high frequency AC power, a step-up transformer for increasing thevoltage of said high frequency AC power to a higher voltage, a rectifierfor converting the higher voltage to charge a charging capacitor to avoltage at or approximately at a command voltage established by saidlaser pulse energy control system.
 13. A laser as in claim 12 whereinsaid power supply is configured to slightly over charge said chargingcapacitor and further comprises a bleed circuit to bleed down saidcharging capacitor to said command voltage.
 14. A laser as in claim 1wherein said pulse compression and amplification circuits furthercomprise an inductor, a pulse transformer and a third capacitor whereinsaid inductor, pulse transformer and said third capacitor are arrangedto permit the high voltage charge on said second capacitor to flow toground through the primary side of said pulse transformer in order toproduce a very high voltage pulse at the output of said pulsetransformer to be stored temporarily on said third capacitor.
 15. Alaser as in claim 14 wherein said primary side of said pulse transformercomprises a plurality of hollow spools, each spool defining an axis,connected in series and a secondary side of said pulse transformer iscomprised of at least one rod co-aligned with the axis of a plurality ofsaid spools.
 16. A laser as in claim 1 wherein said laser pulse powersystem comprises at least one saturable inductor with a coil emersed inoil contained in a pot which also serves as the high voltage lead of theinductor.
 17. A laser as in claim 1 wherein said gas circulatorcomprises a blower comprising a shaft supported by active at least twomagnetic bearings, each bearing comprising a stator and a rotor; saidshaft bearing driver by a motor comprising a stator and a rotor, saidblower also comprising a sealing means for sealing said rotors within anenvironment containing said laser gas with said stator outside saidlaser gas environment.
 18. A laser as in claim 1 wherein said gascirculator comprises a blower comprising a shaft supported by at leasttwo ceramic bearings.